feat!: prepend/append commands to dockerfile

BREAKING CHANGE: Command definition in the configuration has been changed

from

[[docker.commands]]
distro = { id = "Ubuntu" }
command = "echo runs before the base layer"

to

[[docker.commands.prepend]]
distro = { id = "Ubuntu" }
command = "echo runs before the base layer"

config/out-of-tree.go

@@ -35,9 +35,12 @@ type OutOfTree struct {
 		Timeout  artifact.Duration
 		Registry string
 
-		// Commands that will be executed before
-		// the base layer of Dockerfile
-		Commands []distro.Command
+		// Commands that are executed before (prepend) and after (append) the
+		// base layer of the Dockerfile.
+		Commands struct {
+			Prepend []distro.Command
+			Append  []distro.Command
+		}
 	}
 }